Stabilization of polypropylene



United States Patent 3,325,441 STABILIZATION OF POLYPROPYLENE GeorgeJoseph McNally, Montclair, N.J., and Louis Peter Cipriani, Easton, Pa.,assignors to American Cyanamid Company, Stamford, Conn., a corporationof Maine N0 Drawing. Filed Apr. 7, 1964, Ser. No. 358,081 4 Claims. (Cl.260--4l) This invention relates to a method for stabilizingpolypropylene and to the stable compositions thus prepared.

Compositions of polypropylene have found wide application. However, aserious drawback to many uses is their instability upon exposure to heatand light; without stabilization by one means or another, polypropyleneis quickly deteriorated. This vulnerability has hindered use ofpolypropylene for certain applications. If stability could be improved,polypropylene would lend itself to even wider applications.

Various antioxidant materials have been used to stabilize polypropylenewith some degree of success. Until the present time, the need for aneconomical, but effective means of stabilizing has not beensatisfactorily fulfilled.

The present invention is based on the discovery that compositions whichare extremely stable to the effects of heat and light result when thepolypropylene is blended with TiO pigment and a hindered phenol type ofantioxidant. Thus, although each of these additives alone provides onlya small degree of protection against degradation by heat and light, thecombination of additives in the polypropylene results in enhancedprotection even over extended periods of exposure.

It is most surprising and quite unexpected that such effectivestabilization results using the combination of the invention, since, inother plastics (including polyethylene), stabilization by an antioxidantis hardly improved, if at all, by addition of TiO Thus, inacrylonitrile-butadiene styrene, the use of a combination of TiO;,, andan antioxidant shows little increased effect. Also, in polystyrene, forexample, the presence of the TiO -antioxidant combination does notnoticeably increase light stability of the polymer. This lack ofimprovement is also found in polyvinylchloride where the combination ofTiO with antioxidant is no more effective than the latter by itself.

For use in the compositions of this invention, TiO of either the rutileor anatase type may be used. Preferably, the rutile type is used. Theparticle size range of the pigment is not particularly critical, andcommercially available TiO having a range of 0.05 to 0.4 micron isuseful. The concentration of the TiO in the composition may range from0.5 to 5 percent and preferably is in the area of about 2 percent.

The antioxidants which are suitable in the compositions of thisinvention are of the hindered phenol type and include those having oneto three phenolic groups. By hindered phenol is meant a phenol to whichin either one or both of the ortho positions to the hydroxyl group isattached at least one bulky group, for example t-butyl. Specificexamples of the antioxidants are the following:

2,6-di-tert-butyl-4-methylphenol 4,4-thiobis 6-tert-butyl-m-cresol)4,4-butylidenebis 6-tert-butyl-m-cresol)2,6-bis(2-hydroxy-3-t-butyl-5-methylbenzyl)-p-cresol2,2'-methylenebis(4-ethyl-6-tert-butylphenol) 2,2'-methylenebis4-methyl-6-tert-butylphenol) The concentration of the antioxidant in thepolypropylene compositions may range from 0.02 percent to 1.0 percentand, preferably, from about 0.1 to 0.5 percent. A practicalconcentration and one which is preferred is about 0.2 percent.

The invention is further illustrated by the examples which follow.

EXAMPLE 1 Evaluation in polypropylene Polypropylene compositionscontaining the TiO antioxidant combinations are prepared and exposed toultraviolet light. The time of exposure before development ofbrittleness is observed. The procedure is described in more detail asfollows:

Fifty grams of unstabilized polypropylene powder are placed in ascrew-top jar along with the additive to be tested and tumbled for twohours. The blend is then placed on a 6" x 13" mill whose rolls areheated to 360 F. and 320 F. and it is passed through the nip of therolls, stripped, and fed end-wise into the nip to insure maximum mixing.This operation is repeated ten times. From the resulting sheet, a 15 to20 mil sheet is compressionmolded at about 450 F. using abouttwenty-five tons pressure. From this molded sheet, specimens are cut outfor the test.

The specimens are exposed in the Fade-Orneter and outdoors in sunshine.The length of exposure before brittle point is reached, is noted. Theresults of these tests are presented in the following Table I. Forconvenience, the antioxidants are referred to by letter according to thefol lowing code:

TABLE I Time to Brittle Points Hours Months in in Arizona Fade- SunshineOmeter Additive Percent Antioxidant B Antioxidant C Antioxidant DAntioxidant E Antioxidant; F. TlO2 plus Antioxidant A Tl02 AntioxidantO... TiOz.

plus 800 2 Antioxidant D 'liOzplus Antioxidant E TlOz.

plus Antioxidant F TiOzplus Antioxidant B 500 -5 9 9 NP 9 P e ION) N p s2-hydroxy4-octyloxybenzophenone T102 lu s u D Antioxidant B 600 EXAMPLE2 To show that the superior effect of a combination of TiO and anantioxidant occurs in polypropylene but is not found in other well-knownpolymeric plastic compositions, samples ofacrylonitrile-butadiene-styrene (ABS) and polyvinylchloride weresubjected to the following tests.

The additives were combined with the polymer using procedures adapted tothe particular polymer. In the case of the ABS polymer, the number ofhours of exposure in a fluorescent sunlarnp-blacklight lamp (FS-BL)before brittle point was determined. The degree of deterioration inpolyvinylchloride was measured by development of yellow color (change inyellow index) which is an appropriate test for this polymer.

The results are shown in the following tables in which the letters havethe same significance as in Example 1.

TAB LE II.ABS POLYME R Percent Hours Exposure to N o. Antioxidant WeightFS-BL to Brittle Cone. Point 1 With no 'liOz and no 16 antioxidant. Tl02only 2 16 Antioxidant A 0.5 24 Antioxidant A 0.5 24 r x "1i' "ii 3'ntioxi ant .5

ntioxi ant .5 Ti0z &. b.. l 24 Antioxi ant 7 {T102 2.0 l 24 TABLEIIL-RIGID POLYVINYLCHLORIDE Percent Initial Hours Exposure to No.Antioxidant Conc. Yellow I S-BL for Yellow (wt.) Index AYI. 01' 15 1iorgtrolNorxa). 6 350 n ioxi ant 0.2 2 ioau hu n 2. 0 6

ntioxi ant 0.2 3 {T102 2. o l 6 170 We claim:

1. Polypropylene stabilized by the incorporation therein of 0.5 to 5.0percent of TiO and 0.02 to 1.0 percent of a compound selected from thegroup consisting of 2,6-ditert-butyl-4-mgethylphenol,2,6-bis(2-hydroxy-3-t-butyl-5- methylbenzyl) p-cresol,2,2-methylenebis(4-ethyl-6-tertbutylphenol) 2,2 methylenebis(4 methyl6-tert-butylphenol) and 4,4 butylidenebis(6-tert-butyl-m-cresol); saidpercentages being based upon the weight of the polypropylene.

2. The composition of claim 1 wherein the compound is 2,2-methylenebis(4-methyl-6-tert-butylphenol) 3. The composition of claim 1 wherein thecompound is 2,6-di-tert-butyl-4-methylphenol.

4. The composition of claim 1 wherein the compound is 4,4-butylidenebis6-tert-butyl-rn-cresol) No references cited.

MORRIS LIEBMAN, Primary Examiner.

I. S. WALDRON, S. L. FOX, Assistant Examiners.

1. POLYPROPYLENE STABILIZED BY THE INCORPORATION THEREIN OF 0.5 TO 5.0PERCENT OF TIO2 AND 0.02 TO 1.0 PERCENT OF A COMPOUND SELECTED FROM THEGROUP CONSISTING OF 2,6-DITERT-BUTYL-4-METHYLPHENOL,2,6-BIS(2-HYDROXY-3-T-BUTYL-5METHYLBENZYL)-P-CRESOL,2,2H-METHYLENEBIS(4-ETHYL-6-TERTBUTYLPHENOL),2,2''-METHYLENEBIS(4-METHYL-6-TERT-BUTYLPHENOL) AND 4,4''-BUTYLIDENEBIS(6-TERT-BUTYL-M-CRESOL); SAIDPERCENTAGES BEING BASED UPON THE WEIGHT OF THE POLYPROPYLENE.